http://www.eventwax.com is an online event registration tool that uses several techniques for increasing traffic.
1. We use a freemium model and about 75% of our users use the free service to host their free events. This helps us because of #2 below.
2. Our built in viral mechanism is that when one hosts an event, everyone who signs up for an event sees a "powered by eventwax" logo at the bottom of the registration pages. So, even for free events, we are getting a tiny bit of passive exposure to all event's audiences. That means 10's of thousands of people a month see our logo.
3. We have a semi-active blog that brings in some high quality traffic.
4. We've had success at targeting a few keyword phrases that bring in a bit of traffic but SEO targeting has also been one of our biggest disappointments. Online event registration is a very competitive field and we've "wasted" a lot of time trying to improve our keywords.
5. We do run very limited Google Adwords and Linkedin PPC campaigns but haven't optimized them yet. Our current combined budget is only about $100/month. The next think we need to do is "bucket" our keywords into semantic groups and then create custom landing pages for each group to increase relevancy.